The architecture and operating mechanism of a cnidarian stinging organelle

The stinging organelles of jellyfish, sea anemones, and other cnidarians, known as nematocysts, are remarkable cellular weapons used for both predation and defense. Nematocysts consist of a pressurized capsule containing a coiled harpoon-like thread. These structures are in turn built within specialized cells known as nematocytes. When triggered, the capsule explosively discharges, ejecting the coiled thread which punctures the target and rapidly elongates by turning inside out in a process called eversion. Due to the structural complexity of the thread and the extreme speed of discharge, the precise mechanics of nematocyst firing have remained elusive7. Here, using a combination of live and super-resolution imaging, 3D electron microscopy, and genetic perturbations, we define the step-by-step sequence of nematocyst operation in the model sea anemone Nematostella vectensis. This analysis reveals the complex biomechanical transformations underpinning the operating mechanism of nematocysts, one of nature’s most exquisite biological micro-machines. Further, this study will provide insight into the form and function of related cnidarian organelles and serve as a template for the design of bioinspired microdevices.

TRITC WGA TRITC WGA Supplementary Fig. 2 | TRITC and WGA co-staining of discharged capsules and threads. a Images of the capsule, shaft and a fraction of the tubule in Transillumination channel (TI), TRITC (green), WGA (magenta) and overlapping channels of a discharged thread. Arrows indicate overlapping of the shaft filament in each panel. Scale bar 0.5 μm. b Super resolution fluorescent image of a discharged thread showing TRITC labeled material (green, arrows) and WGA labeled material (magenta, dashed arrows in thread and capsules) of a discharged nematocyst. Scale bar 0.5 μm. c Magnified regions of panel b. The differential labeling of the thread by TRITC (green, arrows) and WGA (magenta, dashed arrows) in overlapping and individual channels in distinct sections of the thread. (Representative of purified and discharged threads from ~200 polyps). Scale bar 0.5 μm.  The structure of the shaft and the tubule after knockdown with v1g243188 shRNA. c Strong phenotype showing delaminated shaft after knockdown with v1g243188 shRNA. d Quantified TRITC and WGA-488 fluorescence signal intensity of the shaft filaments after knockdown with scrambled shRNA (n=32 individual threads imaged, purified and discharged from ~300 primary polyps electroporated with shRNA) and v1g243188 shRNA (n=37 individual threads imaged, purified and discharged from ~300 primary polyps electroporated with shRNA). Images were taken at the same settings and background was subtracted. Statistical significance was determined using a two-tailed, unpaired students t-test. Values d=8.9, p=7.54E-12). Bar ± error bar = mean ± SD.

Helix elastic energy
Consider a filament of radius r bent into a helix (no twisting assumed to simplify calculations) with radius R and pitch c defined by the parametric equation The curvature of such a helix is κ = R/(R 2 + c 2 ). The bending energy e per unit length is given by [1] e = πY 0 κ 2 r 4 /8, where Y 0 stands for the filament Young modulus. For tightly coiled structure (its segment is shown in Supplementary Figure 7) we obtain that 2πc c = 6r giving c c = 3r/π ≈ r. The radius of this coil R c = r and the curvature κ c = 1/(2r). Thus we find the bending energy E c of the filament of the length L E c = πY 0 Lr 2 /32 ≈ Y 0 Lr 2 /10.
Consider an everted uncoiled helix with the increased radius R uc = kR c = kr, k > 1 and pitch c uc = mc c = mr, m > 1. Thus, the curvature of this helix reads κ uc = k/((k 2 + m 2 )r) = 2k/(k 2 + m 2 )κ c . This leads to the expression for the bending energy The measurements of the geometric parameters of the coiled and uncoiled helices produce the following values (in µm) R c = r = 0.12, R uc = 0.6, c c = r = 0.12, c uc = 1.2, leading to k = 5 and m = 10. Thus the energy stored in the uncoiled structure is less than 1% of that stored in the coiled structure. This estimate means that nearly all bending energy stored in the inverted structure E b = 3E c is converted into kinetic energy E k = 3E c that can be used for eversion of the shaft-tubule connector and part of the tubule.

Minicollagen fibers material parameters
The staining of the shaft filaments shows that they are made of Ncol1 and Ncol4 in approximately equal proportions. The structural analysis of these proteins exhibits CRD regions, polypropiline segment(s) and more flexible collagen motif. Similar structure is also observed in the CPP-1 molecule found in Hydra nematocyst that has Young modulus of Y = 7.8 ± 8.0 MPa in the bulk [2]. Collagen fibers demonstrate Young modulus values order of few hundreds MPa found using different methods [3,4]. It is convenient to use Y as an estimate for Y 0 . This gives us an estimate for E k for the filaments of the length L = 20 µm E k = 3E c = 0.3Y Lr 2 = 1 pJ.

1
To estimate the energy cost of this process one has to consider the connector as an elastic cylinder of radius r and length L everting inside out. With some assumptions about the tubule elastic properties such a problem has an exact solution for the eversion work A e that reads where µ is the is the shear modulus of the material and δ is the ratio of the inner cylinder radius to the outer one. Taking δ = 0.8 and using shear modulus of the collagen µ = 35 M P a we find A e = 50Lr 2 · 10 −12 J, where the cylinder length L and radius r are measured in microns. Using here r = 0.1 we find A e = 0.5L · 10 −12 J. The length of the connector is L = 5 µm and we finally arrive at A e = 2.5 pJ.
We observe that this energy is comparable to that of provided by the elastic energy of the shaft so that partial eversion of the connector is possible.